When soldering is performed in a reflow furnace (solder reflow), the problem arises that flux components vaporized from a solder paste become a cause of impairing the functions of a cooler for the furnace or a transporting device, or the flux components deposit on a printed circuit board, causing quality deterioration.
Concretely, the following problems with conventional reflow furnaces have been reported.
Patent Document 1 points out that a solvent in a solder paste coated on a printed circuit board in a preheating zone or a main heating zone of a reflow furnace volatilizes, or a solid component such as gum rosin vaporizes, to form a flux fume, which may deposit (as a liquid or a solid) on a relatively low temperature part, thereby impairing the functions of a cooler or a transporting device.
Patent Document 2 points out the following facts: It is difficult to remove flux completely within a heating chamber. When an inert gas containing vaporized flux passes through a narrow passage of a transport section or a board unloading section, and flows out of the furnace, the vaporized flux condenses within the board unloading section or a board discharge section and deposits on the surface of the inner wall. When operation is continued for a long time, the adhering flux drops onto the circuit board, contaminating the circuit board or an electronic component, and causing a defect such as poor performance.
Fluxes comprise vehicles, solvents, activators, and additives. Resins used as the vehicles are reported to be natural rosins (main component: abietic acid) and synthetic resins. As the activators, halogen type and non-halogen type activators are reported. As the solvents, alcohol-based solvents are reported. Concretely, there are the following reports:
Patent Document 3 reports that the resin components incorporated in the paste are natural rosin, disproportionated rosin, polymerized rosin, and modified rosin, the synthetic resins for the paste are polyester, polyurethane, and acrylic resins, the solvents for the paste are alcohol and ether, and the solvents for the paste are alcohol, ether, ester, and aromatic solvents, for example, benzyl alcohol, butanol, ethyl cellosolve, butyl cellosolve, butyl carbitol, diethylene glycol hexyl ether, and dioctyl phthalate.
Non-patent Document 1 reports that organic bromine compounds, for example, 1,2-dibromoethylene, 1,2-dibromoethylene, 1-bromo-2-methylbutane, methyl bromide, and ethylene bromide, and chlorine compounds are present as components which gasify and diffuse during a soldering operation.
Patent Document 4 makes the following report: Typical fluxes contain vehicles, solvents, activators, and other additives. The vehicles are solids or non-evaporable liquids, including rosins, resins, glycols, polyglycols, polyglycol surface active agents, and glycerin. The solvents dissolve the vehicles, activators, and other additives, and evaporate during preheating and soldering. The typical solvents include alcohols, glycols, glycol esters and/or glycol ethers. The activators produce the action of facilitating the removal of metal oxides from the surface of the metal soldered. Generally, the activators include hydrogen chloride compounds of amines; dicarboxylic acids such as adipic acid and succinic acid; and organic acids such as citric acid and maleic acid.
To cope with the above-mentioned problems, various oxidation catalysts for converting the flux components in the furnace gas into CO2 and H2O have been proposed. For example, Patent Document 5 proposes a purification method which comprises causing smoke and a smelly gas generated during reflow treatment within a heating chamber to be acted on by a catalyst provided within a combustion apparatus, thereby removing them, and then refluxing smoke-free heating air within the heating chamber via a piping portion. The catalyst used in this method is an oxidizing granular catalyst such as a platinum-alumina system.
Patent Document 6 proposes a purification method in which an oxidation catalyst is installed in a circulation path for hot air, and a combustible organic gas is supplied to a heating chamber, whereby flux is oxidized and oxygen within the furnace is also consumed. This document discloses that lanthanum, cobalt-based perovskite, platinum, palladium, and rhodium are preferred as the oxidation catalyst.
Patent Document 7 describes as follows: When an oxidation reaction is performed using an oxidation catalyst, necessary oxygen is deficient unavoidably, and the reaction becomes incomplete. To make the catalyst act sufficiently, moreover, a temperature of 300° C. to 400° C. is needed. However, an appropriate temperature condition for soldering is of the order of 250° C., presenting the drawback that matching fails. Thus, this document proposes a purification method comprising providing a catalyst treatment portion charged with an oxidation catalyst outside a furnace body, connecting the catalyst treatment portion to the furnace by piping, and supplying a combustible material and oxygen to the catalyst treatment portion from the outside. The document discloses that as the catalyst, it is preferred to use a porous body formed into a three-dimensional reticulated structure, such as platinum, palladium, lanthanum or rhodium.
Patent Document 8 proposes a method in which a filter, and a catalytic reaction portion for oxidizing flux into CO2, H2O, etc. are provided outside a furnace, and the resulting gases are mixed, unchanged, with an atmospheric gas.
Patent Document 9 proposes a technology in which a furnace gas is passed through a porous material (straightening plate) coated with platinum, a copper-manganese based metal, or a palladium-based metal catalyst, whereby flux components in the gas are chemically combined with oxygen. By so doing, the flux components are decomposed into low molecular weight substances which minimally liquefy, and these substances are treated as an exhaust gas. By this means, flux liquefaction can be prevented, and the oxygen concentration within the furnace can be lowered.
However, even when the above-mentioned oxidation catalysts having high activity are used, the oxygen concentration within the furnace is normally of the order of 0.01 to 3%. This makes it extremely difficult to convert the flux components efficiently into CO2 and H2O. Thus, it is difficult to resolve contamination, etc. within the furnace due to the deposition of the flux components. With treatment with the oxidation catalyst, moreover, generation of carbon monoxide (CO) is unavoidable.
Patent Document 1:JP-A-10-173333
Patent Document 2:JP-A-2003-324272
Patent Document 3:JP-A-11-197879
Patent Document 4:U.S. Pat. No. 6,749,655
Patent Document 5:JP-A-6-14589
Patent Document 6:JP-A-4-371367
Patent Document 7:JP-A-7-204883
Patent Document 8:JP-A-7-212028
Patent Document 9:JP-A-6-114548
Non-patent Document 1: A monthly periodical “Environment and Measurement Technology”, Vol. 25, No. 4, p. 6, 1998 “Handa kanetsu hassei gas no bunseki (Analysis of gas generated upon heating during soldering)”.